Design engineers today are facing more and more challenges. Printed circuit boards are getting more and more complex as physical size at the board, package, and chip level continues to decrease. These structures are also expected to perform at higher speeds. Reducing feature size results in lower costs and faster time to market. However, high density also means more conductive metal layers and more signal traces routed on smaller and thinner boards.
To facilitate electrical communication between conductive layers, a class of well known structures called vias are used. In general, vias are openings etched through dielectric layers between a first conductive layer and a second conductive layer. Vias can be formed by drilling holes and then plating the paths through the holes. The via holes can extend through the complete multilayer board in which case they are called plated through holes. In this configuration, the vias are electrically coupled to each conductive (e.g. copper) layer. As aspect ratios become worse and worse and signal frequencies continue to rise—a trend certain to continue in the future—each signal trace will inevitably exhibit transmission line effects, thereby degrading the signal integrity.
Generally speaking, the parasitic capacitance and inductance of a via hole are small. In fact, in low frequency applications, vias may be constructed between circuit board layers with little or no consideration for the parasitic capacitances and inductances they exhibit. Accordingly, at lower frequencies and slower rise times, the effects of a via or plated through hole on circuit performance are negligible.
However, in high frequency applications, one problem with via structures is the lack of AC electrical isolation between the vias and the remainder of the microcircuit. Another problem is that standard via holes do not provide a controlled impedance environment at high frequencies and, therefore, present discontinuities or changes in impedance along the signal path. Such discontinuities may cause signal reflections (or “ringing”) of high speed digital and analog signals. Specifically, at higher frequencies, such departure from a uniform propagation structure results in parasitic elements being formed at the junctions between the circuit traces and the via itself.
For example, with regard to a standard via structure passing through an entire multilayer board, a small circular or rectangular pad is typically added to each end of the plated through hole so that the plating process that creates the complete via will have some material to which it can anchor to at each end of the structure during fabrication. At sufficiently high frequencies, the small capacitance formed between this via pad and the surrounding metal structures (comprising any signal or ground traces nearby be it on the surface of the board or within buried layers) will form a reactive impedance equal to 1/jωC where C is the capacitance, ω is the frequency in radians per second and j designates the square root of −1. Such reactive impedance will, in turn, result in an impedance mismatch at the trace-to-via junction and energy will, accordingly, be reflected back towards the source end. In addition, the plated through hole itself is a form of inductor in which currents flowing in the longitudinal direction of the via experience an inductive effect due to the basic electromagnetic properties found in wires.
Therefore, the electrical characteristics of a printed circuit board (PCB), used to physically mount and connect circuit components in a high frequency product, will have a significant impact on the performance of that product. In this regard, the term ‘parasitic’ is typically used to refer to a physical attribute of the PCB that has an effect on the performance of the circuit. Accordingly, the potential magnitude of the negative effect of the PCB design on performance increases with frequency as the parasitic elements tend to a similar magnitude to the typical lumped components used.
In general, a standard via structure may be viewed as a transmission line (or waveguide) without sidewalls. The need to move signals between various layers of a printed circuit board, particularly at the higher frequencies used in today's wireless communication systems, has necessitated the creation of unique radio frequency (RF) circuit board structures to compensate for the impedance discontinuities introduced by via holes. Specifically, it would be desirable to realize a transmission line structure capable of transmitting high frequency signals between conductive traces on various layers of a board without degrading the transmission characteristics of such signals. To this end, reducing reflections (or return loss) at the trace-to-via transitions in RF circuit board applications may be achieved using a modified via structure simulating a quasi 50-ohm transmission line to move signals between various circuit layers of a board.